It is common to measure the frequency of electromagnetic radiation by counting the number of radiation cycles which occur during a known time interval. Such frequency counting is not, strictly speaking, a real-time measurement because it gives an average frequency taken over the time interval used. If the frequency should be greater at the beginning of the interval and lesser toward the end of the interval, this approach would not indicate that occurrence.
This fact limits the applicability of such an approach. For example, in an application where frequency measurement is to be used to indicate gas velocity in a jet engine, radiation such as light can be projected onto reflective particles such as dust or soot present in the gas flow. The radiation is reflected by the particles and a change in frequency of the reflected radiation due to the Doppler shift gives an indication of the speed at which the particles are moving and thus the speed of the moving gas. However, the gas can be moving so fast that the particles remain present in the vicinity where the velocity is to be measured for only a very short time before being swept away. Thus, they can reflect for only a similarly short time with the result that the reflected radiation is produced in bursts of extremely short duration. These frequency bursts do not persist for a long enough time to allow frequency counters to function well.
One alternative to utilizing frequency counters is to estimate the frequency by using a bank of bandpass filters spanning a spectrum of interest, each filter occupying a known sub-band of the spectrum. For example, a first filter can have a passband in the range of 0-10 Hertz, a second filter have a band in the range of 10-20 Hertz, and so on, up to a tenth filter having a band in the range of 90-100 Hertz. Thus, a spectrum from 0-100 Hertz would be covered and, for example, a signal from the second filter would indicate that the frequency of an input signal lies somewhere between 10 and 20 Hertz. However, this approach suffers the drawback that only a frequency range and not an exact frequency of the input signal is obtained. Further, the accuracy of the device is determined by the number of filters together with the passband of each. To obtain high accuracy, a large number of filters, each having a narrow passband, is required. Further still, to cover a large spectrum a proportionately larger number of filters is additionally required.